This invention relates, in general, to a method of setting up and controlling synchronisation of a wireline modem, and is particularly, but not exclusively, applicable to the use of pilot tones in modems that support digital subscriber line (xDSL) communication protocols in bidirectional wireline systems.
Telecommunication systems that interconnect wireline subscriber terminals are being developed to support broadband data communication. More particularly, recent developments in broadband communication protocols allow broadband data to be overlaid on narrowband voice or integrated service digital network (ISDN) traffic. Specifically, the interconnection of broadband modems located at the subscriber terminal and at an exchange allow current broadband access systems to communicate on spare spectrum (i.e. spare frequency channels) of a twisted pair communication resource; the spare frequency channels being isolated from conventionally encoded voice signals by a suitable filter. In this respect, and depending upon the complexity of the xDSL coding scheme, overlaid broadband systems can support data rates in excess of two Megabits per second (Mbps), although this rate is dependent upon the physical parameters of the connection, e.g. the overall length of the twisted pair and its composition and configuration.
Asymmetric Digital Subscriber Line (ADSL) and High-speed Digital Subscriber Line (HDSL) protocols, for example, can support data rates of 2 Mbps over distances of approximately three kilometres, while more complex schemes (such as VDSL) can support data rates of 8 Mbps and above over distances of, typically, less than two kilometres. Protocols such as Very high-speed Digital Subscriber Line (VDSL) utilise multiple sub-channel carriers, e.g. in a discrete multi-tone (DMT) environment, to provide an adaptive system that mitigates the effects of cross-talk by selectively ignoring noise-effected sub-channel carriers or reducing the number of bits supported on each sub-channel. As will be appreciated, DMT provides a comb of frequency carriers that are each separated modulated and then combined to generate a composite signal envelope. As such, information (both control information and traffic) is distributed across a number of different frequency carriers.
DMT schemes for supporting VDSL are often realised in a time division duplex (TDD) transmission environment in which a single communication resource, i.e. a frequency band, supports both up-link and down-link transmissions using the same frequencies. In other words, there is a sharing in time of the bandwidth provided by the extended spectrum. The use of guard periods between adjacent groups of time-slots within a TDD frame ensures that rogue overlapping transmissions within the up-link and down-link do not occur, and hence eliminates the likelihood of near-end cross talk (NEXT). In more detail, the guard periods provide a period in which a power amplifier can power-up and power-down, and also allow for some adjustment (i.e. alignment) of the frame with respect to a selected pilot tone on a designated sub-channel carrier of a DMT scheme. NEXT is of particular concern because it occurs when electromagnetic interference is induced into a wireline resource that is communicating information in an opposing direction, e.g. down-link (or downstream) information appears as noise in an up-link (or upstream) path. NEXT is particularly undesirable because near-end generated interference is at a level that can potentially swamp data signals received from a remote terminal, which data signals have previously been subjected to attenuation through the transmission path. Furthermore, NEXT increases significantly the higher frequency components and so in even more undesirable in high frequency data-over-voice wireline systems, such as VDSL.
In order to establish effective end-to-end communication in a communication system, it is necessary for synchronisation between a transmitting unit and an interconnected receiving unit to occur; this is true for both a radio frequency environment and a wireline environment, such a VDSL system employed over a twisted pair. More specifically, synchronisation is required to demodulate encoded signals that are addressed to the receiver. In this respect and in relation to a DMT system (or the like, such as an orthogonal frequency division multiplexed OFDM scheme), a pilot carrier or tone is used in a training sequence on a dedicated (pre-allocated) sub-channel. Initially, upon receipt of the pilot tone, the receiver acquires frequency lock and then establishes phase lock.
At frequency and phase-lock, the equipment does not necessarily have symbol alignment in a TDD scheme and so it is necessary to communicate information in both directions to acquire full synchronisation between, say, line termination equipment (LTE) and customer premises equipment (CPE). For example, while the reception by the CPE of an initial transmission from the LTE can be used to align transmit and receive periods of a TDD frame in the CPE, the guard period between the transmit and receive periods at the LTE must be varied to take into account transmission delays in the communication resource. Therefore, the CPE must send some form of acknowledgement or signal back to the LTE in order that full synchronisation at both ends of the call is established. Even after full synchronisation has been achieved, conventional DMT modems then necessarily use the sinusoidal waveform of the pilot tone as the basis of a phase reference to enable a phase lock loop to correct for any variations in the operating frequency provided by a local crystal oscillator. Unfortunately, a consequence of this approach is that a carrier must be reserved for the pilot function and so channel (data traffic) capacity is therefore restricted.
Another concern in relation to the implementation of wide bandwidth applications (required multi-carrier schemes) arises from the fact that the frequency spectrum seldom provides a homogeneous transmission environment and usually exhibits significant variations in its signal to noise (S:N) ratio across its bandwidth. In fact, portions of the spectrum are often unusable as a consequence of the presence of interference, such as noise and intermodulation products. To avoid these poor spectral regions, modems associated with the transmitter and receiver negotiate for the best sub-channel carriers during training and reject those sub-channel carriers that have a performance below a predetermined and acceptable threshold. Unfortunately, however, since the unmodulated pilot tone on its dedicated sub-channel is fundamental in establishing and maintaining synchronisation, present systems must invariably fix this sub-channel and therefore inherently subject the pilot to noise and general interference problems that either preexist or arise during a call in the same spectral region as the pilots sub-channel. For example, the lines over which a modem will operate will be subject to many sources of noise, such as radio frequency interference, cross-talk and impulse noise. In addition, bridge taps on the loop can create notches in the transmission characteristics of the loop. Consequently, present systems of the prior art are occasionally unable to locate the pilot for training purposes and are therefore unable to establish a call. Additionally, prior art systems may loose the pilot during a call, which loss results in a loss of synchronisation and consequently both the inability to recover encoded information and the earlier termination of the call.
Clearly, it would be desirable for a multi-carrier system to have immunity to spectral variation, especially in relation to pilot tones.
According to a first aspect of the present invention there is provided a method of establishing synchronisation between a transmitting unit and an addressed unit over a communication resource supporting a multiplicity of sub-channel carriers, the method comprising the steps of: from the transmitting unit, sending a time-continuous pilot tone on each of a plurality of predetermined sub-channels; detecting at least one of the time-continuous pilot tones at the addressed unit and assessing a quality of the at least one time-continuous pilot tone; based on the quality, selecting a time-continuous pilot tone; and acquiring frequency and phase lock between the transmitting unit and the addressed unit using the selected time-continuous pilot tone.
In another aspect of the present invention there is provided a modem arranged to operate in a time division duplexed multi-carrier environment, the modem comprising: a controller for regulating the operation of the modem and a memory coupled to the controller and having a look-up table identifying a plurality of sub-channels on each of which a time-continuous pilot tone is initially transmitted; wherein the controller comprises: means for regulating transmission of the plurality of time-continuous pilot tones; means for assessing quality of incident time-continuous pilot tones; means for selecting a time-continuous pilot tone based on quality; and means for acquiring frequency and phase lock using the selected time-continuous pilot tone.
The present invention therefore advantageously provides a multi-carrier system that is generally more robust and specifically less susceptible to initial or time-varying spectral variation that effect sub-channels, especially those used in relation to a pilot tone for training and reference.